Optimized reflector stacks for solidly mounted bulk acoustic wave resonators

The quality factor (Q) of a solidly mounted bulk acoustic wave resonator (SMR) is limited by substrate losses, because the acoustic mirror is traditionally optimized to reflect longitudinal waves only. We propose two different design approaches derived from optics to tailor the acoustic mirror for effective reflection of both longitudinal and shear waves. The first one employs the stopband theory in optics; the second one takes advantage of the periodic nature of reflection spectra in a Bragg reflector: the diffraction grating design approach. The optimized design using stopband theory reaches a calculated minimum transmission of −25 dB and −20 dB at resonance frequency for longitudinal and shear waves, respectively, for various practical reflector material combinations. Using the diffraction grating approach, a near quarter-wave performance is maintained for longitudinal waves, whereas shear waves reach minimum transmission below −26 dB. However, this design does necessitate relatively thick layers. The experimental results show good agreement with finite element models (FEM).\ud The extracted 1-D Q for the realized shear optimized devices was increased to around 3300.\u

Validity of the parabolic effective mass approximation in silicon and germanium n-MOSFETs with different crystal orientations

This paper investigates the validity of the parabolic effective mass approximation (EMA), which is almost universally used to describe the size and bias-induced quantization in n-MOSFETs. In particular, we compare the EMA results with a full-band quantization approach based on the linear combination of bulk bands (LCBB) and study the most relevant quantities for the modeling of the mobility and of the on-current of the devices, namely, the minima of the 2-D subbands, the transport masses, and the electron density of states. Our study deals with both silicon and germanium n-MOSFETs with different crystal orientations and shows that, in most cases, the validity of the EMA is quite satisfactory. The LCBB approach is then used to calculate the values of the effective masses that help improve the EMA accuracy. There are crystal orientations, however, where the 2-D energy dispersion obtained by the LCBB method exhibits features that are difficult to reproduce with the EMA model

Barrier Height Variation in Ni-Based AlGaN/GaN Schottky Diodes

In this paper, we have investigated Ni-based AlGaN/GaN Schottky diodes comprising capping layers with silicon-Technology-compatible metals such as TiN, TiW, TiWN, and combinations thereof. The observed change in Schottky barrier height of a Ni and Ni/TiW/TiWN/TiW contact can be explained by stress effects induced by the TiW/TiWN/TiW capping layer, rather than by chemical reactions at the metal-semiconductor interface. Secondary-ion mass spectroscopy and transmission electron microscopy techniques, for samples with and without a TiW/TiWN/TiW cap, have been used to show that no chemical reactions take place. In addition, electrical characterization of dedicated samples revealed that the barrier height of Ni/TiW/TiWN/TiW contacts increases after stepwise selective removal of the TiW/TiWN/TiW cap, thus demonstrating the impact of strain

Opportunities for shear energy scaling in bulk acoustic wave resonators

An important energy loss contribution in bulk acoustic wave resonators is formed by so-called shear waves, which are transversal waves that propagate vertically through the devices with a horizontal motion. In this work, we report for the first time scaling of the shear-confined spots, i.e., spots containing a high concentration of shear wave displacement, controlled by the frame region width at the edge of the resonator. We also demonstrate a novel methodology to arrive at an optimum frame region width for spurious mode suppression and shear wave confinement. This methodology makes use of dispersion curves obtained from finite-element method (FEM) eigenfrequency simulations for arriving at an optimum frame region width. The frame region optimization is demonstrated for solidly mounted resonators employing several shear wave optimized reflector stacks. Finally, the FEM simulation results are compared with measurements for resonators with Ta2O5/SiO2 stacks showing suppression of the spurious modes

Piezoelectric strain modulation in FETs

We report on a feature for the transistor, a piezoelectric layer to modulate the strain in the channel. The strain is proportional to the gate-source voltage, and thus increases as the device is turned on. As a result, the device has the leakage current of a relaxed device and the lower threshold voltage of a strained device. Our results, obtained by combining electrical and mechanical simulations, demonstrate that strain modulation can result in a 9 mV/decade smaller subthreshold swing for a FinFET

Electrostatic Doping in Semiconductor Devices

To overcome the limitations of chemical doping in nanometer-scale semiconductor devices, electrostatic doping (ED) is emerging as a broadly investigated alternative to provide regions with a high electron or hole density in a semiconductor device. In this paper, we review various reported ED approaches and related device architectures in different material systems. We highlight the role of metal and semiconductor workfunctions, energy bandgap, and applied electric field and the interplay between them for the induced ED. The effect of interface traps on the induced charge is also addressed. In addition, we discuss the performance benefits of ED devices and the major roadblocks of these approaches for potential future CMOS technology

Electron-hole bilayer light-emitting device: Concept and operation

We report a novel switched-mode light-emitting device (LED) in an undoped ultra-thin-body (UTB) based on the electrostatically-induced electron-hole bilayer (EHB) concept. The proposed device works on the principle of formation of EHB channels by applying suitable gate biases during the charging-cycle, and their recombination during a discharging-cycle. Using TCAD simulations, we show that continuous switching of the gates in an indium arsenide (InAs) based EHB LED with a ~12 μs time period leads to radiative recombination of the induced charge carriers with a peak internal quantum efficiency (IQE) as high as ~92% and a time-averaged IQE of ~29%. The proposed concept obviates the need for chemically doped p-n junctions in the UTB device for light-emitting applications. However, when relying on the thermal generation alone as a source of charge carriers in a small undoped semiconductor volume, a narrow bandgap semiconductor (such as InAs) is required for the proposed LED which ultimately limits the switching speed. For wider bandgap materials, highly doped regions on either side of the intrinsic UTB layer in the form of a lateral PIN structure could be employed where switching speed is then not limited by thermal generation. TCAD simulations of a silicon (Si) EHB LED based on such a gated PIN structure shows switching capability in the GHz frequency range making it attractive for SOI based optocoupling applications